Various furnaces have been devised for low temperature processing of ores, however, many of them have been inefficient due to substantial heat losses and also due to a lack of efficiency in transferring heat from the fire box to the ore as it passes downwardly through the furnace. Accordingly, many prior art or processing furnaces have been inefficient due to improper transfer of heat to the ore and loss of heat from the furnace due to radiation conduction or convection of such heat away from the furnace and away from the ore being processed therein.